1. Field of the Invention
The present invention pertains to a method for molding plastic ophthalmic lenses and more particularly to an improved step for filling the mold cavity.
2. Description of the Prior Art
Plastic lenses for eye glasses have traditionally been manufactured using well known molds and molding technics.
Classically, there is used a mold consisting of two pieces that are separated and sealed by either a tape or a gasket for defining a molding cavity.
A liquid polymerizable monomer composition is introduced in the mold cavity by means of a filling nozzle passing through the tape or gasket. Filling of the mold cavity can be effected either from the top or the bottom of the mold.
Examples of such mold and mold filling methods are disclosed in U.S. Pat. No. 3,070,746, U.S. Pat. No. 4,693,446, U.S. Pat. No. 5,547,618 and U.S. Pat. No. 5,662,839.
These molds and filling methods ate effective and used in the industry.
However, when the mold cavity is filled up with the liquid polymerizable monomer composition from the top of the mold, air bubbles are often entrapped within the liquid composition resulting in defectuous lenses. This entrapping of air bubbles is particularly pronounced when molding minus lenses.
In fact, the mold cavity for molding minus lenses is thinner at its central portion than at its peripheral portion and due to the thinness of the mold central portion it is likely that air bubbles are entrapped within the liquid composition in this central portion and cannot escape from the composition.
Thus, the aim of the invention is to provide a method for molding plastic lenses using a two-piece mold which is filled by the top and avoiding entrapping of air bubbles within the liquid polymerizable monomer composition filling the mold cavity.
Accordingly, the present invention contemplates a method for molding a plastic ophthalmic lens using a two-piece mold, said two-piece mold defining a molding cavity having a vertical central axis which is part of a fixed reference system comprising said vertical axis and a horizontal central axis which intersects at the center of the mold, said horizontal central axis dividing the molding cavity into a lower half and an upper half, and comprising a filling nozzle for introduction of a liquid polymerizable monomer composition into the molding cavity and situated at the periphery of the upper half of the mold cavity, said method comprising:
(a) starting introduction of the liquid polymerizable monomer composition into the molding cavity with the filling nozzle spaced apart from the vertical central axis of the mold cavity by a distance sufficient so that the liquid polymerizable monomer composition flows in a region of the molding cavity adjacent to the periphery thereof;
(b) continuing introduction of the liquid polymerizable monomer while the filling nozzle is spaced from the vertical central axis until at least the lower half of the molding cavity is filled up;
(c) moving relatively from each other the filling nozzle and the two-piece mold to bring the filling nozzle in register with the vertical central axis of the molding cavity;
(d) continuing introduction of the liquid polymerizable monomer composition while the filling nozzle is as in step (c) until the molding cavity is fully filled;
(e) stopping introduction of the liquid polymerizable monomer composition and polymerizing the composition; and
(f) recovering the polymerized ophthalmic lens.
Preferably, in steps (a) and (b), the filling nozzle is spaced apart from the vertical central axis of the molding cavity by a distance at least equal to R/2, more preferably 3R/4 or more, where R is the radius of the molding cavity.
In step (c), although displacement of the two-piece mold relative to the filling nozzle can be effected either by tilting the two-piece mold or by moving the filling nozzle, it is preferred to do it by tilting the two-piece mold.
Relative displacement of the two-piece mold and the filling nozzle can be effected while continuing introduction of the liquid polymerizable monomer composition. It is also possible to stop introduction prior to step (c) and to restart introduction once the filling nozzle is in register with the vertical central axis of the molding cavity.